全文获取类型
收费全文 | 60121篇 |
免费 | 1692篇 |
国内免费 | 198篇 |
专业分类
电工技术 | 724篇 |
综合类 | 645篇 |
化学工业 | 8116篇 |
金属工艺 | 890篇 |
机械仪表 | 1254篇 |
建筑科学 | 1817篇 |
矿业工程 | 436篇 |
能源动力 | 893篇 |
轻工业 | 3684篇 |
水利工程 | 711篇 |
石油天然气 | 104篇 |
武器工业 | 5篇 |
无线电 | 3380篇 |
一般工业技术 | 6339篇 |
冶金工业 | 24317篇 |
原子能技术 | 371篇 |
自动化技术 | 8325篇 |
出版年
2023年 | 327篇 |
2022年 | 337篇 |
2021年 | 540篇 |
2020年 | 353篇 |
2019年 | 409篇 |
2018年 | 868篇 |
2017年 | 1048篇 |
2016年 | 1525篇 |
2015年 | 1185篇 |
2014年 | 930篇 |
2013年 | 1457篇 |
2012年 | 2940篇 |
2011年 | 3483篇 |
2010年 | 1411篇 |
2009年 | 1551篇 |
2008年 | 1443篇 |
2007年 | 1364篇 |
2006年 | 1245篇 |
2005年 | 3895篇 |
2004年 | 3010篇 |
2003年 | 2457篇 |
2002年 | 1251篇 |
2001年 | 1021篇 |
2000年 | 585篇 |
1999年 | 957篇 |
1998年 | 6861篇 |
1997年 | 4257篇 |
1996年 | 2880篇 |
1995年 | 1727篇 |
1994年 | 1324篇 |
1993年 | 1355篇 |
1992年 | 423篇 |
1991年 | 455篇 |
1990年 | 436篇 |
1989年 | 408篇 |
1988年 | 414篇 |
1987年 | 334篇 |
1986年 | 310篇 |
1985年 | 304篇 |
1984年 | 201篇 |
1983年 | 195篇 |
1982年 | 216篇 |
1981年 | 278篇 |
1980年 | 275篇 |
1979年 | 155篇 |
1978年 | 184篇 |
1977年 | 724篇 |
1976年 | 1455篇 |
1975年 | 178篇 |
1974年 | 112篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
In rhodopsin, the 11-cis-retinal chromophore forms a complex with Lys296 of opsin via a protonated Schiff base. Absorption of light initiates the activation of rhodopsin by cis/trans photoisomerization of retinal. Thermal relaxation through different intermediates leads into the metarhodopsin states which bind and activate transducin (Gt) and rhodopsin kinase (RK). all-trans-Retinal also recombines with opsin independent of light, forming activating species of the receptor. In this study, we examined the mechanism by which all-trans-retinal activates opsin. To exclude other amines except active site Lys296 from formation of Schiff bases, we reductively methylated rhodopsin (PM-rhodopsin), which we then bleached to generate PM-opsin. Using spectroscopic methods and a Gt activation assay, we found that all-trans-retinal interacted with PM-opsin, producing a noncovalent complex that activated Gt. The residual nucleotide exchange in Gt catalyzed by opsin was approximately 1/250 lower relative to that of photoactivated rhodopsin (pH 8.0, 23 degrees C). Addition of equimolar all-trans-retinal led to an occupancy of one-tenth of the putative retinal binding site(s) of opsin and enhanced the Gt activation rate 2-fold. When the concentration of all-trans-retinal was increased to saturation, the Gt activation rate of the opsin/all-trans-retinal complex was approximately 1/33 lower compared to that of photoactivated rhodopsin. We conclude that all-trans-retinal can form a noncovalent complex with opsin that activates Gt by different mechanisms than photolyzed rhodopsin. 相似文献
92.
Immunofluorescence microscopic observations indicated that a monoclonal antibody, Vmp 18, raised against the peptide 199-208 of murine interleukin 1 alpha, cross-reacted with an antigenic determinant of Drosophila thorax muscles. Immunoelectron microscopic analysis showed that the gold particles were mainly localized in the Z-line which is the attachment site of thin filaments from adjacent sarcomeres. On the contrary, the antibody failed to mark the Z-line in vertebrate skeletal muscle. A Western blot of total protein extract from Drosophila thorax muscles bound a protein of 43 kDa. Our observations suggest that the Vmp 18 antibody could contribute to clarify the composition of the Z-line in insect's flight muscles. 相似文献
93.
F Ildan S Polat AI G?cer A Oner T Isbir UO Mete M Kaya A Karadayi 《Canadian Metallurgical Quarterly》1996,138(3):338-345
The sodium-potassium activated and magnesium dependent adenosine-5'-triphosphatase (Na(+)-K(+)/Mg(+2) ATPase EC.3.6.1.3.) activity and lipid peroxidation and early ultrastructural findings were determined in rat brain at the acute stage of ischaemia produced by permanent unilateral occlusion of the middle cerebral artery (MCA). The effects of the pretreatment with intravenous high-dose methylprednisolone (MP) on these biochemical indices and ultrastructural findings were also evaluated in the same model. The rats were divided into four groups. In group I, 10 rats were used to determine Na(+)-K(+)/Mg(+2) ATPase activity and the extent of lipid peroxidation by measuring the malondialdehyde (MDA) content and normal ultrastructural findings. In group II on 20 rats, only subtemporal craniectomy was done in order to determine the effects of the surgical procedure on these indices and findings. This group was treated intravenously with saline solution before occlusion. In group III with MCA occlusion, saline solution was administered intravenously to 20 rats in the same amount of methylprednisolone used in group IV, ten minutes before the occlusion. In Group IV, a single high-dose (30 mg/kg) of methylprednisolone was administered intravenously, ten minutes before occlusion in 20 rats. After occlusion of the middle cerebral artery, Na(+)-K(+)/Mg(+2) ATPase activity was decreased promptly in the first ten minutes in the ischaemic hemisphere and remained at a lower level than the contralateral hemispheres in the same group and the normal levels in group I, during 120 minutes of ischaemia. A single dose methylprednisolone pretreatment prohibited the inactivation of Na(+)-K(+)/Mg(+2) ATPase. On the other hand, there was significant difference in malondialdehyde content between group I and group III. Malondialdehyde levels were significantly increased following ischaemia and a non-significant increase was observed in the contralateral hemisphere. Methylprednisolone treatment significantly decreased malondialdehyde content on the side of the ischaemic hemisphere. We conclude that there is a positive relationship between membrane-bound enzyme Na(+)-K(+)/Mg(+2) ATPase activity, malondialdehyde content and early ultrastructural changes in the treated group with MP. These data suggest that the pretreatment injection of high doses (30 mg/kg) methylprednisolone contribute to the protection of the brain from ischaemia with stabilization of the cell membrane by effecting the lipid peroxidation and the activation of Na(+)-K(+)/Mg(+2) ATPase. 相似文献
94.
95.
96.
By nature, cryptographers tend to be extremely cautious and conservative. From their perspective, spending time getting people to use a new cryptographic algorithm is roughly on par with getting involved in a land war in Asia. In some cases, however, the difficulty involved in developing a new algorithm is worth the effort. This is the case with identity-based encryption (IBE), a new type of public-key algorithm 相似文献
97.
Steel box sections are usually fabricated from flat plates which are welded at the corners. The welding process can introduce residual stresses and geometric imperfections into the sections which can influence their strength. For some thin-walled sections, large periodic geometric imperfections have been observed in manufactured sections. Subsequent investigations have indicated that the imperfections are in fact buckling deformations i.e. the box section has buckled due to welding residual stresses prior to any application of external load. The welding procedure and the behaviour of the box sections under load has been modelled using a finite element analysis that accounts for both geometric and material non-linearities. Tests have been carried out on box sections with a range of width to thickness ratios for the plate elements. Modelling has been shown to give good correlation with the test results. The conditions for buckling to take place as a result of the welding process have been established. A design method has been proposed. 相似文献
98.
99.
100.